Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.
Two common types of flash memory array architectures are the “NAND” and “NOR” architectures. These architectures are named for the resemblance that the basic memory cell configuration of each architecture has to a basic NAND or NOR gate circuits, respectively.
FIG. 1 illustrates a simplified diagram of a typical prior art NAND flash memory array. The memory array of FIG. 1, for purposes of clarity, does not show all of the elements typically required in a memory array. For example, only two bit lines are shown (BL0 and BL1) when the number of bit lines required actually depends upon the memory density.
The array is comprised of an array of floating gate cells 101 arranged in series strings 104, 105. Each of the floating gate cells 101 are coupled drain to source in each series chain 104, 105. A word line (WL0-WL31) that spans across multiple series strings 104, 105 is coupled to the control gates of every floating gate cell in a row in order to control their operation. The bit lines BL0, BL1 are eventually coupled to sense amplifiers (not shown) that detect the state of each cell.
In operation, the word lines (WL0-WL31) select the individual floating gate memory cells in the series chain 104, 105 to be written to or read from and operate the remaining floating gate memory cells in each series string 104, 105 in a pass through mode. Each series string 104, 105 of floating gate memory cells is coupled to a source line 106 by a source select gate 116, 117 and to an individual bit line (BL0, BL1) by a drain select gate 112, 113. The source select gates 116, 117 are controlled by a source select gate control line SG(S) 118 coupled to their control gates. The drain select gates 112, 113 are controlled by a drain select gate control line SG(D) 114.
A selected word line 100 for the flash memory cells 130-131 being programmed is typically biased by programming pulses that start at a voltage of around 16V and may incrementally increase to more than 20V. The unselected word lines for the remaining cells are typically biased at Vpass. This is typically in an approximate range of 9-10V. The bit lines of the cells to be programmed are typically biased at 0V while the inhibited bit lines are typically biased at VCC.
As NAND flash memory is scaled, parasitic capacitance coupling between adjacent memory cell floating gates becomes a problem. Floating gate-to-floating gate interference can cause a wider Vt distribution when the distribution is needed to be tighter. The wider distributions can result in degraded programming performance as well as other problems.
These problems for single level cell (SLC) NAND arrays are even worse in a multiple level cell (MLC) array. MLC memory stores multiple bits on each cell by using different threshold levels for each state that is stored. The difference between adjacent threshold voltage distributions is typically very small as compared to an SLC memory device. Therefore, the effects of the floating gate-to-floating gate coupling in an MLC device are greatly increased.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a way to minimize the effects of floating gate-to-floating gate coupling in a memory device.